Field of Invention
The invention relates to methods and systems for generating in the interior of a medium or body radiant energy for producing a change in the properties of a medium or body by exposure to the radiation.
Discussion of the Background
Presently, light (i.e., electromagnetic radiation from the radio frequency through the visible to the x-ray and gamma ray wavelength range) activated processing is used in a number of industrial processes ranging from photoresist curing, to on-demand ozone production, to sterilization, to the promotion of polymer cross-linking activation (e.g. in adhesive and surface coatings) and others. Today, light activated processing is seen in these areas to have distinct advantages over more conventional approaches. For example, conventional sterilization by steam autoclaving or in food processing by pasteurization may unsuitably overheat the medium to be sterilized. As such, light activated curable coatings are one of the fastest growing sectors in the coatings industry. In recent years, this technology has made inroads into a number of market segments like fiber optics, optical and pressure-sensitive adhesives, and automotive applications like cured topcoats, and curable powder coatings. The driving force of this development is mostly the quest for an increase in productivity of the coating and curing process, as conventional non light activated adhesive and surface coatings typically require 1) the elimination of solvents from the adhesive and surface coatings to produce a cure and 2) a time/temperature cure which adds delay and costs to the manufacturing process.
Moreover, the use of solvent based products in adhesive and surface coatings applications is becoming increasingly unattractive because of rising energy costs and stringent regulation of solvent emissions into the atmosphere. Optimum energy savings as well as beneficial ecological considerations are both served by radiation curable adhesive and surface coating compositions. Radiation curable polymer cross-linking systems have been developed to eliminate the need for high oven temperatures and to eliminate the need for expensive solvent recovery systems. In those systems, light irradiation initiates free-radical cross-linking in the presence of common photosensitizers.
However, in the adhesive and surface coating applications and in many of the other applications listed above, the light-activated processing is limited due to the penetration depth of light into the processed medium. For example, in water sterilization, ultraviolet light sources are coupled with agitation and stirring mechanisms in order to ensure that any bacteria in the water medium will be exposed to the UV light. In light-activated adhesive and surface coating processing, the primary limitation is that the material to be cured must be directly exposed to the light, both in type (wavelength or spectral distribution) and intensity. In adhesive and surface coating applications, any “shaded” area will require a secondary cure mechanism, increasing cure time over the non-shaded areas and further delaying cure time due to the existent of a sealed skin through which subsequent curing must proceed (i.e., referred to as a cocoon effect).